1. Field of Invention
The present invention relates to broadband wireless access systems and amongst other things to a method of dynamic maintenance of quality of service in a broadband wireless access system.
2. Discussion of Background
Point to multi-point fixed broadband wireless access systems over MMDS networks are known in broadcast situations. These networks operate over licensed bands including the MMDS band (2,150 to 2,162 MHz), the WCS band (2,305 to 2,360 MHz) and the ITFS/MMDS bands (2,500 to 2,686 MHz).
A known cable based broadband access system, which operates at a range of between 50 MHz and 864 MHz, but not in the MMDS, WCS, or ITFS/MMDS bands, is the data over cable specification system, which is specified in the data over cable system interface specifications (DOCSIS). An overview of a cable based DOCSIS system is depicted in FIG. 1. A CMTS 10 communicates with a wide area network 20, such as the internet. The CMTS 10 can transmit signals from the wide area network 20 along a cable network 30 through cable modems 40 to CPE 50 (Customer Premise Equipment—intended throughout this document to include a computer and/or all of the equipment at the customer site, such as a LAN—Local Area Network). CPE 50 messages can be transmitted to the wide area network 20 through the cable modem 40 along the cable network 30 to the CMTS 10.
In point to multi-point broadband access systems one central end-point, e.g. the head-end, communicates through a bi-directional link or links with multiple end-points, e.g. the nodes. The number of nodes in communication varies in time and can be none, one or two or more at any specific time.
The link(s) between the head-end and the nodes are combined in one or more channels. The signal path from the central end-point to the nodes is referred to as downstream, while the signal path from the nodes to the central end-point is referred to as upstream.
A single upstream channel can be used to deliver information from a node to the head-end, and a downstream channel is used from the head-end to a node or a group of nodes. If a single upstream channel is used for communication from the nodes(s) to the central point, then only one end-point can sends information on the single upstream channel at any one time.
A known allocation scheme, for scheduling upstream channels and mini-slots in the channels, is referred to as contention-based based allocation. This allocation scheme allows more than a single node to use the same time interval. In such allocation an allocation scheme, there is some probability that more than one node will try to send information on the same upstream channel at the same time. In this case, the information from all or some of the nodes transmitting messages on the same upstream channel at the same time will not be received at the central end-point. These nodes, from which the message is not received, will retransmit the same message until such a time when the central end-point receives that transmission. Further, during the time when it is re-transmitting the same message, the node cannot transmit new messages.
Another known upstream channel allocation scheme is defined DOCSIS. These specifications refer to the case of HFC network. In the DOCSIS system, each upstream channel is assigned a different frequency range. Different channels are used for upstream or downstream directions.
In the DOCSIS scheme there is no need to coordinate the downstream channel, since only the head-end is transmitting in this direction. Further in DOCSIS, the head-end is responsible for the allocation of the upstream channels. These allocations are performed in two general steps, one for the allocation of an upstream channel and the other for the allocation of time intervals in the upstream channels.
The allocation of the time intervals on each upstream channel is also performed by the head-end. The head-end transmits the time interval allocations on the downstream channel in a message called MAP. A single MAP message describes time interval allocation on a single upstream channel for a specific period of time.
The DOCSIS solution uses a fixed upstream channel for each node, which implies that statistical changes to the traffic load may cause a high load on one channel, while not allowing other channels with lower loads to be used to balance higher load channel. Further, if the performance of the current upstream channel of a node becomes unacceptable, e.g. falls below predetermined threshold levels, the node must switch to an alternate channel. This switching process, which includes a search for the best available channel, takes a longtime during which service to the node is interrupted.
When multiple services are supported by the broadband access system, some Quality of Service (QoS) requirements need to be defined which add additional limitations on any allocation scheme or scheduling scheme. The services may include voice IP, broadband video on demand or other services that may require different downstream and upstream bandwidth, with respect to the modem, than the standard IP traffic that comprises Internet communication.
Therefore, it is necessary to schedule upstream communication based upon the specific requirements of each service that is being utilized by the user.